In recent years, in office automation equipment, such as word processors, personal computers, facsimile machines, various measuring instruments, such as medical measuring instruments, and other devices, ink jet printers have been extensively used for printing information from these devices at high density. As is well known in the art, in the ink jet printer, an ink droplet is ejected from a head section of the printer and deposited directly onto a recording medium, such as recording paper, to perform monochrome or color printing. The ink jet printer has many advantages including that printing can be performed on even a three-dimensional recording medium, running cost is low since plain paper can be used as the recording medium, the head can be easily mounted in the printer, the need to provide the step of transfer, fixation and the like can be eliminated, color printing is easily performed, and a sharp color printed image can be provided. The head section of the ink jet printer can be classified into several types according to the drive system for ejecting ink droplets from the head section. Among them, a typically and advantageously used one is a piezoelectric ink jet head.
The piezoelectric ink jet head generally comprises: a plurality of ink chambers which are disposed at equidistant spaces and function as an ink flow passage and a pressurizing chamber for ejecting an ink; and a nozzle plate mounted on the front end of the ink chambers and equipped with nozzles, for ejecting an ink, corresponding respectively to the ink chambers; and pressing means for pressurizing an ink within the ink chamber in response to the demand for printing. The pressing means comprises a piezoelectric element (known also as "piezo element"), and electrostrictive effect attained by this piezoelectric element is utilized to create a pressure wave within an ink chamber, filled with ink, in the head section, permitting the ink to be ejected through the nozzle in the head section.
The structure of the piezoelectric ink jet head will be described in more detail with reference to FIG. 1. An ink jet head 10, a part of which is shown in the drawing, has an ink chamber member 11 comprising a plurality of ink chambers 12 serving as an ink flow passage and a pressurizing chamber for ejecting ink. A nozzle plate (not shown) equipped with nozzles disposed so as to correspond respectively to the ink chambers 12 is mounted on the front end of the ink chamber 11. The ink pressurized within the ink chamber 12 can be ejected as a droplet through the bore of the nozzle. In the ink chamber member 11 shown in the drawing, pressing means is mounted on the open face of the ink chamber 12. In the example shown in the drawing, the pressing means comprises: a diaphragm 15 for creating a change in volume of the ink chamber 12; a piezoelectric element 17 as a driving element for distorting the diaphragm 15; and an upper electrode 16 and a lower electrode 18 which can apply voltage according to need, the piezoelectric element 17 being sandwiched between the upper electrode 16 and the lower electrode 18.
Ferroelectric elements have been extensively used as the piezoelectric element for the ink jet head or as an element, for example, for capacitors, actuators, memories and other elements. The ferroelectric element consists essentially of a ferroelectric or a ferroelectric material. Typical ferroelectric materials include an oxide ceramic represented by the general formula ABO.sub.3 and having a simple perovskite structure as shown in FIG. 2 and an oxide ceramic having a composite perovskite structure represented by the general formula (A.sub.1, A.sub.2, . . . ) (B.sub.1, B.sub.2, . . . )O.sub.3. The term "perovskite structure" used herein refers to both a simple perovskite structure and a composite perovskite structure unless otherwise specified. As shown in the drawing, a ceramic having the above perovskite structure contains metallic ions A and B in the structure. Examples of more specific ferroelectric materials having the above structure include lead zirconate titanate (PZT) represented by the general formula Pb(Zr, Ti)O.sub.3. In particular, ferroelectrics, containing lead (Pb) as one metal component, including PZT are generally known to have large remanence, specific permittivity, and piezoelectric constant and possesses excellent piezoelectricity and ferroelectricity. In the present specification, the ferroelectric material will be described particularly with reference to PZT.
A sol-gel process has hitherto been well known as a technique for the production of PZT, particularly PZT in a thin layer form. Use of the sol-gel process in the production of PZT is advantageous in that a high-purity thin layer of PZT can be formed, the composition of the formed thin layer of PZT can reflect the composition of the starting material used, which facilitates the control of the composition and can provide a thin layer of PZT having high surface smoothness by repetition of spin coating and firing.
The production of a thin layer of PZT by the sol-gel process and use of the thin layer of PZT as a piezoelectric element will be described in more detail. For example, as described in Japanese Unexamined Patent Publication (Kokai) No. 6-112550, lead acetate is dissolved in acetic acid, and the solution is heated under reflux for 30 min. Zirconium tetrabutoxide and titanium tetraisopropoxide are then dissolved in the solution, water and diethylene glycol are added dropwise thereto, and the mixture is satisfactorily stirred to conduct hydrolysis. To the resultant alcohol solution of a PZT precursor is added polyethylene glycol monomethyl ether in an amount of 10% by weight based on the PZT precursor, followed by satisfactory stirring. Thus, a homogeneous sol is prepared. A platinum electrode is formed on a silicon substrate, the sol is then spin-coated onto the electrode, and the coating is heated at about 350.degree. C. Thus, a 2.5 .mu.m-thick, thin, crack-free porous gel layer can be formed.
Subsequently, the same starting material as the above PZT material is hydrolyzed to form a sol. In this case, however, no polyethylene glycol monomethyl ether is added. The sol is spin-coated onto the above thin, porous gel layer to form a coating which is then dried by heating at 400.degree. C. The thin layer thus formed is fired in an oxygen atmosphere for 15 hr. The firing temperature is generally 600 to 700.degree. C. Thus, a thin layer of PZT having a perovskite structure can be formed through the above series of steps. The above sol-gel reaction may be represented by a general formula as shown in FIG. 3 wherein R represents an alkyl group.
Further, hydrothermal synthesis has hitherto been well known as a method for the formation of a thin layer of PZT from an aqueous solution of a main starting compound. The formation of the thin layer of PZT by hydrothermal synthesis will be described. For example, as described in Japanese Unexamined Patent Publication (Kokai) No. 6-112543, 0.2 mol of lead nitrate, 0.104 mol of zirconium oxychloride, and 0.096 mol of titanium tetrachloride are dissolved in a 2 N aqueous potassium hydroxide solution. A silicon substrate with a platinum electrode provided thereon is immersed in the solution, and the system is heated in an autoclave at 160.degree. C. for 30 hr. The substrate is taken out of the autoclave and dried at 200.degree. C. for one hr to form a thin layer of PZT of cubic particles having an average diameter of 5 .mu.m.
Alternatively, the hydrothermal synthesis may comprise a seed crystal formation process and a crystal growth process. At the outset, in order to form a seed crystal of PZT, a titanium substrate is immersed in water containing lead hydroxide Pb(OH).sub.2 and zirconium hydroxide Zr(OH).sub.4, and the system is heated in an autoclave at a temperature of 140 to 200.degree. C. This heating results in the formation of a thin layer of PZT, capable of serving as a seed crystal for subsequent layer formation, on the surface of the titanium substrate. After the formation of the seed crystal, the titanium substrate is immersed in water containing Pb(OH).sub.2, Zr(OH).sub.4, and titanium hydroxide Ti(OH).sub.4, and the system is heated in an autoclave at a temperature of 80 to 150.degree. C. The heating results in the formation of a layer of PZT, in a coarse particle form, having a larger thickness than the thin layer of PZT formed above on the thin layer of PZT.
The formation of the thin layer of PZT using the hydrothermal synthesis has advantages including that the layer thickness can be increased at a low temperature of 200.degree. C. or below and an additional step, which renders the process complicated, is unnecessary since the piezoelectricity is developed immediately after the layer formation, and the adhesion to the substrate is excellent.
The above conventional methods for producing a ferroelectric material, however, involves many problems to be solved. For example, in the method, described in Japanese Unexamined Patent Publication (Kokai) No. 6-112550, wherein the thin layer of PZT is formed by the sol-gel process using a metal alkoxide as a main starting compound, use of the alcohol as a solvent for the PZT precursor poses a problem that the viscosity of the precursor varies depending upon the moisture content of the air, leading to the occurrence of uneven properties of the formed thin layer of PZT. Further, in order to avoid the adverse effect of the moisture in the air and, therefore, to avoid the formation of insolubilized metal alkoxide, the starting compounds should be mixed together in a specific atmosphere, so that the handling of the starting compounds is not easy. Furthermore, in the sol-gel process, it is difficult to increase the thickness of the thin layer of PZT.
This is true of the hydrothermal synthesis. For example, for the thin layer of PZT formed by the hydrothermal synthesis described in Japanese Unexamined Patent Publication (Kokai) No. 6-112543, the average diameter of PZT particles constituting the thin layer is so large that the surface smoothness of the layer is low and it is difficult to form the upper electrode thereon. Further, the hydrothermal synthesis involves problems which include the density of the thin layer being low due to coarse PZT particles and that potassium (K) is left in the thin layer, adversely affecting the properties.
The present inventors have further made extensive and intensive studies and, as a result, have found that even use of water instead of the alcohol according to the present invention creates problems in some cases.
As described below in detail, according to the present invention, preferably, three metal salts or metal alkoxides, lead nitrate (Pb(NO.sub.3).sub.2), zirconium oxynitrate (ZrO(NO.sub.3).sub.2), and titanium isopropoxide (Ti(O-i-C.sub.3 H.sub.7).sub.4), are used as the starting compound to prepare an aqueous PZT precursor solution. The aqueous PZT precursor solution is coated onto a predetermined substrate, and the PZT coating is dried and fired to form a thin layer of PZT. In this case, a first possible problem is association of lead in the step of drying the PZT precursor coating. In general, when components, which are different from each other in solubility in water, are mixed together to prepare an aqueous solution, the components contained in the solution associate with each other in the course of the subsequent step of drying the solution. This phenomenon occurs also in the step of drying the PZT precursor coating, and the association of lead is significant. As a result, there is a possibility that a material is significantly precipitated in the form of crossed stripes on the surface of the thin layer of PZT. More specifically, this is apparent from a microphotograph (magnification: 20.times.) shown in FIG. 4. The creation of the association of lead is considered to result in not only the creation of undesired defects on the surface of the thin layer but also other problems connected with the defects.
In PZT ceramics, it is known that, regarding the ratio of components constituting the PZT ceramics, a Pb:Zr:Ti:O ratio of 1:0.53:0.47:3 offers the highest piezoelectric properties and, when the ratio deviates from this ratio, the piezoelectric properties are rapidly deteriorated. Therefore, in the thin layer of PZT thus formed, even when the Pb:Zr:Ti:O ratio is the above desired value in the stage of the precursor, the association of lead created in the course of drying leads to an undesired variation of the ratio, resulting in the formation of an uneven layer which provides deteriorated piezoelectric properties. In addition, this lowers the density of the layer.
A second possible problem is the creation of defects such as cracks or pinholes. For example, coating of the aqueous PZT precursor solution onto a substrate by a conventional method, such as dip coating or spin coating, followed by drying, degreasing, and firing to form a thin layer of PZT often creates cracks when the thickness of the thin layer is 1 .mu.m or more. The creation of cracks could not be avoided even though the thin layer of PZT is formed by stacking a plurality of thinner layers on top of each other or one another. The creation of cracks in the thin layer of PZT results in lowered density of the layer, making it impossible to form an element, such as an electrode, on the top of the layer. Therefore, the formed thin layer of PZT cannot be used, for example, as a piezoelectric element of an ink jet head. Further, since the aqueous PZT precursor solution used in this case has a low viscosity on the order of several centipoises, the coverage per coating is small and, in addition, pinholes and the like are likely to occur.
In view of the importance of the problem of the creation of cracks or pinholes, the present inventors have made an experiment and, as a result, have found that coating of an aqueous PZT precursor solution having a composition with the Pb:Zr:Ti:O ratio being 1:0.53:0.47:3 (the above described preferred ratio) onto a substrate by dip coating followed by drying at 150.degree. C. develops the formation of protrusions of a material in a crossed stripe form. After the subsequent firing at 700.degree. C. for crystallization, the protrusions were present, and no noticeable disappearance of the protrusions was observed. EDX (energy dispersive X-ray analysis) has revealed that the material in a stripe form constituting the protrusions has a high lead content. Further, when an aqueous solution of lead, an aqueous solution of zirconium, and an aqueous solution of titanium were prepared as described above, dropped on a substrate and allowed to stand at room temperature, a crystal was precipitated only in the aqueous lead solution.
The thickness of the thin layer of PZT formed from the aqueous PZT precursor solution is 0.05 .mu.m at the largest per coating step, and, in addition, pinholes were created. Further, when a series of steps of coating, drying, degreasing, and firing were repeated ten times, the thickness of the formed thin layer of PZT was 0.5 .mu.m, and, in addition, cracks were created.